There are many by-products and natural products which have a value and/or utility limited partly or largely by the contaminants included therein. The nature and amount of the contaminant can, in cases, eliminate all substantial uses of the product, which often results in the product being transported to, and dumped in, a landfill.
For example, the positive economic and technical benefits of utilizing fly ash as a replacement for cement in concrete have been well established. Further, it is well known that ever increasing environmental regulations on coal-fired plants has led to the development of several types of fly ash beneficiation processes designed to make a product suitable for utilization.
The greatest barrier to higher utilization rates for coal fly ash is the heterogeneous nature of fly ash itself. The single greatest cause of heterogeneity within coal fly ash stems from residual unburned carbon, which remains in the fly ash in varying amounts—depending on the efficiency of the coal combustion process that produced the fly ash. These unburned carbon particles are typically agglomerations of devolatized coal char and partially vitrified inorganic inclusions. These tiny inorganic inclusions would have become separate, discrete fly ash particles if the coal had been able to burn completely when originally fired in the boiler.
Because coal fly ash is primarily inorganic mineral matter, the preferred uses for fly ash intend to take advantage of the specific nature and characteristics of the mineral matter—e.g., reactive glass, spherical morphology, particle size distribution, etc. There are significant differences between the nature and character of the mineral matter and the residual carbon in fly ash, and these differences affect the performance of fly ash; particularly, the differences in density, color, surface chemistry, adsorption, etc.
Consequently, in some applications that may benefit from the unique character of the mineral matter in fly ash, the very presence of any unburned carbon negatively affects the performance of the resulting product. This results in a reduction in the utilization rates of fly ash. Additionally, due to differences in the combustion techniques and/or efficiency of different coal-fired boilers and differences in the composition of the coal being burned, inconsistent amounts of unburned carbon in the raw fly ash product further limits the applications for the fly ash and results in still lower utilization rates for fly ash.
There are other causes of heterogeneity in fly ash that are often cited as barriers to increased beneficial use of fly ash, especially when used to produce non-cementious products, such as plastics, rubber, paints, adhesives, etc. Some of those causes are:                Contaminants (e.g., ammonia, activated carbon, etc.) introduced through the combustion and pollution control processes and which are collected in the fly ash;        Variable ash chemistry and color from coal switching/blending;        Agglomerates of mineral matter from high temperature and/or fouling; and        Unburned organics and/or variable ash chemistry from co-firing with alternative fuels, such as biomass and pet coke.        